To make the plates from Rockland fit your current 4x5 holders, just trim them down with a heavy pair of sizzors or with a pair of tin snips. Cut them slightly smaller than the long side of the holders. Leave them longer than the area to be exposed. When you put them in the holders, slide one end under the top edge of the holder and then close the bottom of the holder to keep the plate in place.
You can trim them to the same size as a sheet of film and with a little work they will slide in like a sheet of film. Personally, I find getting them out after exposure a hassel, so I trim them smaller than a sheet of film.